Method for the powder metallurical forming of metal powders by hot casting



1967 A. BUDINCSEVITS 3,351,464

MING OF METAL METHOD FOR THE POWDER METALLURGICAL FOR POWDERS BY HOT CASTING Filed July 25, 1966 [III WU] INVENTOR AND we .15 uA/Ncst'w 7.5

BY A

ATTORNEYS United States Patent 3,351,464 METHOD FOR THE POWDER METALLURICAL FORMING OF METAL POWDERS BY HOT CASTING Andor Budincsevits, Budapest, Hungary, assiguor to Tavkozlesi Kutato Intezet, Budapest, Hungary Filed July 25, 1966, Ser. No. 567,778 Claims. (Cl. 75-212) ABSTRACT OF THE DISCLOSURE Metal powders, of high melting metals and rare earth metals, are shaped by powder metallurgy by coating the metal powder with 1% by weight of a mixture of oleic acid and paraflin wax at about 80 C. to enhance the wettability of the powder by parafiin wax. Thereafter the coated powder is mixed with about 10% by weight of paraflin wax at about 80 C. to form a fluid paste. The fluid paste is then pressed into a cold =mold, under fluid pressure of about 2-6 atmospheres. The molded object is then heated to drive off the paraflin, presintered, and finally sintered.

The manufacture of metal components and objects by the use of conventional powder metallurgy techniques is limited to a very great extent by the geometrical shape and size limitations of the tools for powder pressing. The conventional techniques are often uneconomical by the necessity of using hard metal pressing tools. A further disadvantage of the powder pressing t chnique is the uneven distribution of density in the pressed object, which is due to the non-homogenous distribution of pressure inside the pressing tool. This may result in the appearance of layer structure and cracks in the green pressings. The manufacture of large size objects, particularly of powders of refractory metals, is very expensive due to the need for high power hydraulic presses. Moreover, conventional powder metallurgy techniques based on powder pressing are unsuitable for the fabrication of objects having certain geometries, such as tubes, vessels, cups, dishes, crucibles and the like.

The present invention eliminates the above described disadvantages of the conventional techniques, and makes powder metallurgy techniques available for applications beyond their present scope. The invention is suitable for the manufacture of objects from powders of refractory metals and their mixtures, as well as mixtures of metals of different melting points, and mixtures of metal and metal oxide powders. The invention is suitable for the manufacture of objects of any geometrical shape and size, with uniform distribution of density.

The process according to the invention is as follows: The metal powder or powders are first coated with a thin organophylic layer, which will be described later in this specification. Owing to this the addition of small amounts of a plasticizing material is sufiicient to produce a paste of the metal powder which is fluid at a temperature not higher than 200 C.-300 C. In the case of par-afiin wax as plasticizer for example, this temperature is 80 C. According to the invention, the said paste is pressed by the superatmospheric pressure of air or an inert gas through an injection nozzle into a composite, demountable metal tool which forms the negative of the object and which can be made of a soft metal, such as brass or mild steel. The applied superatmospheric pressure may be a few atmospheres. The warm metal paste fills the cavity inside the tool, solidifies when cooled down and adopts its shape. After a few seconds the tool can be opened, the object can be removed from the tool and is of suificient mechanical strength for further processing. In the case of castthe process of cooling and solidifying may be accelerated by water cooling of the tool.

According to the invention the plasticizing material is removed by evaporation from the finished cast object. This is accomplished by the embedding of the object in an oxide powder which is wetted by the plasticizer, for example, aluminium oxide. In the example of paraflin wax as plasticizer, the evaporation takes place in 68 hours by heating to a temperature of 200 C. The evaporation will not be completed; approximately 0.51.0% of the plasticizer will be left in the object to enable further processing. Alternatively the object is left embedded in the powder and pre-sintered at a high temperature, in wet hydrogen. In most cases, it is, however, preferable to do the final sintering after removing the object from the oxide powder. The sintering temperature will depend on the material properties of the metal powder. If the above described procedure is carried out correctly, the shrinkage observed during sintering will be of the same order as the shrinkage of sintered objects prepared by conventional powder metallurgy techniques. By the application of the technique specified by the above invention, the tolerance in shrinkage will be no more than i0.1%. In cases Where higher accuracy is required than this, the dimensions may be adjusted by grinding.

Experience indicates that in the process as described the powder particles are exposed to a uniform hydrostatic pressure and are closely packed according to the colloidal properties of the individual particles. This is shown by the fact that the sintering temperature required is lower by 50-100 C. than with objects prepared by conventional powder metallurgy techniques.

The first stage of the procedure according to the invention is the preparation of the metal powder, or powders. The metal powders are heated up, depending upon their material properties, to a temperature of 100 C.-300 C., to remove moisture adsorbed to the surface of the particles. The powder is then cooled down to C. and a mixture of oleic acid and paraflin wax of 1% by weight of the metal powder is added. This procedure is carried out in a porcelain ball mill pre-heated to a temperature of C. The powder-plasticizer mixture is rotated in the ball mill for one hour, during which the warm paraffin wax and oleic acid vapour produces a thin coating on the surface of the particles. The powder, which was pretreated in the above manner is then transferred to a crucible, and at the temperature of approximately 80 C. 10% paraffin wax is added. A fluid paste is thus obtained. This paste, when cooled to room temperature, will solidify. The paste prepared is then transferred into the apparatus shown in the attached drawing.

The machine used for casting according to the present invention consists of a stainless steel container covered by a lid which hermetically seals the container. Through the center of the lid 3. stainless steel tube extends into the container which does not touch the bottom of the container so that the paste should be able to penetrate inside this tube. Outside the lid, the tube ends in a conical injection nozzle. The container is placed in a heat insulated vessel, which can be heated from the bottom (for example electrically) to the required temperature. On the side of the container an inlet tube is mounted, through which gas pressure can be applied to the warm paste inside the container. This gas inlet line is fitted with an air inlet, which is an on/oif valve, a manometer, an air admittance valve, and an adaptor to the compressor. The casting mold, which is formed by the tool, is placed on the pouring nozzle and clamped there either mechanically or pneumatically in such manner that it shall withstand the excess pressure of air or gas used in the process of casting. In the attached drawing: 1 represents the container, 2 is a stainless steel tube extending into the coning larger size objects,

tainer, 3 is a heat insulated vessel and heater, 4 is the pouring nozzle, 5 is the casting mold, 6 is a mechanical clamping device, 7 an air admittance valve, 8 the on/ off valve for the air inlet, 9 is a manometer, 10 is a nonreturn ball valve, 11 is an adaptor to the compressor.

The casting of various objects is as follows: The casting tool containing the mold is placed on the pouring noz- Zle and clamped. The buffer vessel is filled by gas of 2-6 atmospheres pressure, depending on the size and shape of the object. The air inlet valve is opened, by which pressure is exerted on to the surface of the warm paste inside the container. As a result of the presure the paste penetrates into the cold mold through the steel tube and pouring nozzle. The paste takes up the shape of the casting mold and in contact with the cold tool will cool and solidify. After closing of the air inlet valve and removal of air pressure by opening of the air admittance valve, the tool can be removed from the injection nozzle. The casting spigot, i.e. the small rod at the bottom of the casting which assumes the shape of the pouring nozzle, can be removed by cutting off with a sharp knife.

The objects cast according to the above procedure will be put through the subsequent treatments of evaporation of the plasticizer, and sintering, as described above.

I claim:

1. A powder metallurgical process, comprising coating metal powder with a film of a mixture of oleic acid and paraffin wax of about 80 C. to enhance the wetability of the powder by paraffin wax, thereafter admixing the coated powder with hot parafiin wax to form a fluid paste, and thereafter pressing the fluid paste into a mold.

2. A process as claimed in claim 1, said mold being at about room temperature.

3. A process as claimed in claim 1, said hot paraffin wax being used in a quantity of about 10% by weight of the coated powder.

4. A process as claimed in claim 1, in which at the time said paste is pressed into said mold, said mold is at a temperature at which said paste is solid, so that the paste strikes relatively cold mold walls.

5. A process as claimed in claim 1, in which said pressure is exerted directly on said paste by a fluid at a pressure of about 2-6 atmospheres.

6. A process as claimed in claim 1, in which said filmforming mixture of oleic acid and paraffin is used in an amout about 1% by weight of the metal powder.

7. A process as claimed in claim 1, said paste being at a temperature about 80 C. when pressed into the mold.

8. A process as claimed in claim 1, and thereafter driving the wax from the molded material by heating the molded material at a temperature about 200 C.

9. A process as claimed in claim 8, the molded material being embedded in an oxide powder that is wetted by paraffin, during the time the paraffin is driven off by heating.

10. A process as claimed in claim 9, said oxide powder being aluminum oxide.

References Cited UNITED STATES PATENTS 1,873,223 8/1932 Sherwood --212 2,744,011 5/1956 Samuel 75222 X 2,792,302 5/1957 Mott 75-222 X 2,928,733 3/1960 Wagner 75-222 X 3,001,871 9/1961 Thien-Chi 75222 X 3,142,892 8/1964 Powell 75222 X 3,234,308 2/1966 Herrmann 264328 X 3,253,066 5/1966 Hardy 264-328 X 3,273,977 9/1966 Davis 75-.-201 X 3,296,353 1/1967 Novel 264328 X FOREIGN PATENTS 580,414 9/ 1946 Great Britain.

L. DEWAYNE RUTLEDGE, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

A. J. STEINER, Assistant Examiner. 

1. A POWDER METALLURGICAL PROCESS, COMPRISING COATING METAL POWDER WITH A FILM OF A MIXTURE OF OLEIC ACID AND PARAFFIN WAX OF ABOUT 80*C. TO ENHANCE THE WETABILITY OF THE POWDER BY PARAFFIN WAX THEREAFTER ADMIXING THE COATED POWDER WITH HOT PARAFFIN WAX TO FORM A FLUID PASTE, AND THEREAFTER PRESSING THE FLUID PASTE INTO A MOLD. 